Mahalanobis-type balance criterion from Morgan & Rubin (2015). $$m = \frac{n_t n_c}{n} (\bar{X}_t - \bar{X}_c)' \Sigma^{-1} (\bar{X}_t - \bar{X}_c)$$
Mahalanobis-type balance criterion from Morgan & Rubin (2015). $$m = \frac{n_t n_c}{n} (\bar{X}_t - \bar{X}_c)' \Sigma^{-1} (\bar{X}_t - \bar{X}_c)$$